In combustion systems which generate heat for industry, power plants, incineration of waste, and the like, fuels such as natural gas, coal, fuel oil, and other fuels are used. To produce the heat from these combustion systems, these fuels are typically burned in compressed air or oxygen enriched compressed air or substantially pure oxygen obtained by gas separation. Because of conservation concerns and the increasing cost of fuel prices and increasingly stringent pollution control, these systems must be thermodynamically efficient and must limit their release of pollutants into the atmosphere within the increasingly stringent control levels. Cost and complexity of the systems and cost of operation are consequently skyrocketing to meet these conditions. A great amount of the cost is when air is used as the oxygen supply. Since air is 75.6% nitrogen, means to remove extremely large amounts of NOx from the emissions such as by scrubbing becomes necessary. When the fuel contain large amounts of contaminants such as sulfur, then removal of those contaminants such as SO2 by utilizing a fluidized bed or by a flue gas desulfurization process such as by absorption is necessary. Equipment for such removal is expensive and its use results in substantial parasitic power loss and inefficiency.
High temperature combustion systems are necessary for industrial applications such as glass, steel, aluminum, paper and pulp industry and cement manufacturing, to name a few industrial applications. Incineration of municipal wastes also require high temperature combustion systems. High temperature is necessary to decompose or at least make molten materials for processing or disposal. Consequently, high temperatures have been used to dispose of waste. Oxygen-enriched air or substantially pure oxygen is sometimes used for multi-staged combustion or high temperature processes which usually result in fuel savings, production increase and reduced waste processing. Oxygen-enriched air increases the adiabatic temperature of the flame thus increasing the local radiative heat transfer. Further, it reduces the mass fraction of nitrogen. and thus reducing NOx emissions. But a large air separation unit which produce gaseous oxygen is expensive or cost prohibitive and the power consumption of this process can represent around 50 percent of the overall production costs. New chemical and refining processes, and the economies of scale of such processes, will require increasing quantities of gaseous oxygen at a single location. Requirements for 15,000 tons per day or more, of gaseous oxygen delivered at pressures of 1,250 psia or higher are anticipated for such processes.
In all of these systems a large amount of heat can be lost in the flue gases. To reduce energy loss, heat recovery systems are used that capture heat of the flue gasses and transfer it to another medium directly or such as through a working fluid to perform useful work as mechanical energy, electrical energy, chemical energy, and the like. One way useful work is achieved is passing the working fluid through a turbine to generate electricity. This working fluid can also be used for other processes. Economy can be achieved by transferring the heat back into the combustion fuel or preheating the load material. In all of these systems there is a remainder of the flue gas that must be extensively and very costly treated for pollutants and exhausted into the atmosphere at pollution levels within governmental controls.
U.S. Pat. No. 6,637,183 issued to Viteri et. al., for example, discloses a stationary power plant which utilizes a semi-closed Brayton Cycle Gas Turbine Power System that can convert an open combined cycle gas turbine into a reduced or zero emissions power system. The system includes a compressor which compresses air and combusts the air with a hydrocarbon fuel. The products of combustion and the remaining portions fo the air from the exhaust is expanded through a turbine. The turbine drives the compressor and outputs power. The exhaust exits the turbine and then is routed through a heat recovery steam generator. A bottoming cycle portion of the system includes a gas generator which combusts a hydrocarbon fuel with oxygen. Water is also entered into the gas generator where it is heated and combined with the products of combustion before entering a bottoming turbine. The water is then separated and routed back to the gas generator after preheating within the heat recovery steam generator. This system depicts the extent of costly equipment and extent of treatment needed to deal with the ordeal of the use of air as the supply of oxygen.
It is desirable to have a combustion system that would be an economical closed combustion system that eliminated pollutants. Avoiding the use of air for the oxygen supply would be necessary. Using contaminant free fuel or means of economically eliminating the contaminant from the fuel will also be necessary. Heretofore, the most pollution free combustion systems have been a closed Brayton cycle using nuclear power, solar or geothermal heat source to heat a working fluid. Because that working fluid is not exhausted, it would not be a source of atmospheric pollution. Those heat sources are renewable non-polluting heat sources so that atmospheric emissions are avoided. However, these systems suffer from drawbacks which have limited their ability to be fully competitive with hydrocarbon fuel combustion systems.